1. Field of the Invention
The present invention relates to an apparatus and a process for holding an object such as a substrate and a wafer, and in particular, to an apparatus and a process for holding a substrate suitable to be used in a proximity exposure apparatus or aligner.
2. Related Background Art
In recent years, there has been a growing need to improve refined working techniques in semiconductor device or IC fabrication apparatuses as the capacity of semiconductor memories is enhanced. However, in order to improve the refined working techniques of the IC fabrication apparatuses, an enhancement of alignment accuracy between a wafer and a mask as well as a high resolution of a light source is required.
In order to achieve the high resolution of a light source, it is possible to use a synchrotron radiation light as a light source in place of a far ultra violet radiation and the like conventionally used in exposure apparatuses (see, for example, Japanese Laid-open (Kokai) No. 2-100311). On the other hand, in order to attain the enhancement of the alignment accuracy, it is necessary to compensate for process strains resulting from ion milling, etching and the like as well as to reduce a positional detection error by the improvement of accuracy in an alignment system for performing the alignment between a wafer and a mask.
In the light of the above facts, following systems or processes have been presented as a process for vacuum-holding a wafer in which the refined working techniques of the IC fabrication apparatus are improved.
(1) Process strains are compensated for by independently controlling temperatures of a wafer and a mask to expand or contract them. Thus, the process strains are compensated for by a magnification correction (see, for example, Japanese Laid-open No. 53-98782).
(2) Air is caused to flow between a wafer and a mask to generate an air curtain. Hence, the temperature of at least one of the wafer and the mask is controlled to expand or contract them, and a magnification correction is conducted. As a result, process strains are compensated for (see, for example, Japanese Laid-open No. 55-123131).
(3) After a wafer is vacuum-held by a wafer chuck, the vacuum-holding is once released. Temperatures of the wafer and the wafer chuck are then made equal to each other (see, for example, Japanese Publication (Kokoku) Nos. 1-14703 and 1-52898).
Following problems, however, occur in a case where each of the above-discussed wafer vacuum-holding processes is applied to an X-ray exposure apparatus using the above-mentioned synchrotron radiation light as a light source which is one of proximity type exposure apparatuses.
(1) In the process disclosed in the Japanese Laid-open No. 53-98782, when the wafer and the mask are arranged with a very narrow proximity gap (normally 10 to 100 .mu.m) therebetween as in the above-discussed X-ray exposure apparatus, the temperature of the mask will also be changed if that of the wafer is varied. Therefore, it is difficult to independently change their temperatures.
(2) In the process disclosed in the Japanese Laid-open No. 55-123131, a proximity gap is extremely small. As a result, it is difficult to create the air curtain between the wafer and the mask. Further, there is a possibility that the mask will be bent.
(3) In the processes disclosed in the Japanese Publication Nos. 1-14703 and 1-52898, a thermal strain of the wafer can be removed. However, since the correction of a magnification cannot be performed, it is impossible to compensate for the process strains.